Water Protection
Introduction
Some woods have natural durability
against decay (rot). Others can be made durable through treatment with
preservatives. Durable species such as redwood and cedar are commonly used for
wood exposed outdoors, such as siding, shakes and shingles, decks, furniture,
and fences. Durability is imparted by natural chemicals, which are contained in
extractives in the heartwood of these species. Heartwood is the center part of
the wood; the outer part is sapwood. Since only the heartwood contains
extractives, lumber that contains a proportion of sapwood does not have the
natural durability of lumber that contains a high proportion of heartwood.
Nondurable wood species may be
factory-treated with preservative for long-term durability for use in ground
contact. These treatments are done in large cylinders, and the preservative
chemicals are forced deep into the wood using high pressure. Preservative
chemicals include creosote, pentachlorophenol, ammoniacal copper zinc arsenate (ACZA),
ammoniacal copper quaternary compound (ACQ), and chromated copper arsenate (CCA).
Wood treated with CCA is one of the most common types of preservative-treated
lumber available today. Several other preservatives are currently used and still
others are under development. Preservative treatments of wood are done under
carefully controlled factory conditions, and the wood usually has a
manufacturer's guarantee. A number of wood preservatives certify their treatment
with a tag on each piece of lumber.
Water repellents (WRs) and
water-repellent preservatives (WRPs) are penetrating wood finishes that increase
the durability of wood by enabling the wood to repel liquid water. This ability
to repel liquid water is imparted by a wax, an oil or a similar water-repelling
substance. By repelling water, WRs and WRPs enable wood to resist decay and
discoloration by wood-decay fungi, which need moisture to live. The addition of
a fungicide to the water repellent, which converts the WR to a WRP, further
enhances the effectiveness of the finish by inhibiting the growth of mildew and
decay fungi. Water repellents and WRPs also decrease the swelling and shrinking
that lead to cracking and warping. They protect the painted wood from
blistering. Cracking, and peeling. In wood species that contain colored
water-soluble extractives, such as redwood and cedar, WRs and WRPs help reduce
the discoloration caused by extractive bleed.
Wood is the material of choice for
many structures. As with any building material, the use of wood depends on its
properties, such as strength and stiffness, as well as the finishing
characteristics and maintenance requirements. Problems such as poor finish
performance, mildew, checking and splitting, and wood decay can be controlled
with proper care and maintenance. Such problems can be avoided or attenuated
through knowledge about the factors that affect wood, particularly wood exposed
outdoors. If wood structures are given proper care initially and are maintained
periodically, they can be functional and structurally sound, as well as
aesthetically pleasing, for decades.
This report includes a discussion
of the effects of outdoor exposure on wood, characteristics of WR and WRP
formulations, and methods for applying WRs and WRPs.
Effects Of Outdoor Exposure
Properly seasoned wood that stays
dry is not subject to decay, premature failure of paints and finishes, and
problems associated with weathering, such as excessive splitting and checking,
raised grain, extractive bleed, and discoloration.
Moisture Effects
Water is one of wood's worst enemies. Whether in the form of vapor or liquid,
water can cause shrinking and swelling, which can lead to dimensional changes of
the wood and degradation of the finish. Water causes decay or rot of the wood
and early failure of paint, and it accelerates the weathering of wood exposed
outdoors.
Shrinking and Swelling - In
general, wood shrinks as it loses moisture and swells as it gains moisture. More
precisely, wood changes dimension between an absolutely dry state (completely
free of moisture) and its fiber saturation point (the point at which the wood
fibers are completely saturated with moisture). This fiber saturation point
typically occurs at about 30% moisture content for most species. At this point,
all the wood's water is bound within the cell wall. At moisture content changes
above fiber saturation, the cell cavities take on or lose unbound water but the
wood cell walls do not change dimensionally. Below the fiber saturation point,
however, the wood will change dimension with changing moisture content. The
magnitude of this change is dependent on species and is always different for the
three axes: radial, tangential, and longitudinal. A large percentage of wood
finish degradation (e.g., paint defects, peeling, and cracking) results from
moisture changes in the wood and subsequent dimensional instability.
Water Vapor-Water Effects -
Shrinking and swelling of wood occur whether the water is in the form of vapor
or liquid. For example, wood swells during periods of high humidity and shrinks
during periods of low humidity; it also swells and shrinks as it gets wet from
liquid water, then dries. As discussed, wood can swell until it reaches fiber
saturation. If wood is exposed to water vapor, such as occurs indoors, the
moisture content can only reach the fiber saturation point. This requires
exposure to 100% relative humidity for an extended period. Since wood is seldom
exposed to this level of relative humidity for long periods, it seldom reaches
fiber saturation because of high humidity. However, if the wood gets wet from
liquid water, it can quickly reach, or even go beyond, fiber saturation.
Problems with poor performance of wood occur when the moisture content of wood
reaches or goes beyond fiber saturation -- this is almost always caused by
liquid water. Throughout the remainder of this report, the term water refers
only to liquid water, water vapor to humidity, and moisture to both water and
water vapour.
Weathering
Regardless of the care taken in building a structure, wood ages when exposed
outdoors. This aging process is called weathering. Weathering is a degradation
of the wood's surface caused by the combined effects of the ultraviolet (UV)
radiation in sunlight, water, and abrasion by wind-blown sand or other
particulates. This degradation should not be confused with decay.
Weathering is first manifested by
a change in the color of the wood. The color of most preservative-treated lumber
is either light green (from copper and chromium salts in the preservative) or
brown (from added dye). Cedar and redwood have the freshly-sawn natural color of
these species. With weathering, dark wood, such as redwood and cedar, tends to
get lighter, whereas light wood, such as pine and fir, tends to get darker. In
some climates, such as along the seashore, wood tends to weather to a silvery
gray. This color is a combination of mildew growth and cellulose enrichment of
the surface. The weathering process removes the colored extractives and lignin,
leaving cellulose. If protected from excessive moisture, redwood and cedar are
prone to weather to silver-gray. Wood that has been pressure-treated with CCA
but not treated with a WRP will initially turn to dull gray. Eventually, this
wood will also become silver-gray.
The change in color is followed by
a loosening of wood fibers and gradual erosion of the wood surface. Rain washes
the degraded wood materials from the surface. Rain and/or changes in humidity
also cause dimensional changes in the wood that accelerate this erosion process.
Erosion is more rapid in the less dense earlywood than in the latewood, which
leads to an uneven surface. Surface erosion, however, proceeds slowly. The
erosion rate for solid softwoods in temperate zones is on the order of 1/4 to
1/2 inch (6 to l2 mm) per century and depends mainly on the intensity of UV
radiation and on the wood species. For hardwoods, the erosion rate is 1/8 to 1/4
inch (3 to 6 mm) per century. The erosion rate depends on the exposure of the
wood to sun and rain and the care the wood receives. Control of water absorption
by the wood retards weathering and decay.
Extractive Bleed
A common cause of discoloration is extractive bleed. All species contain
extractives, but extractive bleed is more prevalent on highly colored woods. The
discoloration often occurs around fasteners because the hole in the wood caused
by the fastener cuts many wood cells. These cut cells increase water absorption.
Water dissolves the extractives, and when the wood dries, the extractives
accumulate at the surface and sunlight causes them to polymerize. Although
extractive bleed can be a problem on wood siding, it is seldom a problem on
horizontal wood surfaces such as decks because the extractives are usually
washed from the deck by rain before they polymerize. If extractive bleed is a
problem, the extractives can be removed by scrubbing the wood with soap and
water. Do not use a wire brush because the brush will contaminate the surface
with iron which will cause iron stain. Finishing wood with a WRP greatly
minimizes extractive bleed.
Iron Stain
A common form of staining on wood surfaces resufts from contamination with iron.
A portion of the extractives in wood includes a group of chemicals collectively
called tannins. The amount of tannins depends on species; oak, redwood, and
cedar are rich in tannins. Tannins react with iron to form a blue-black stain on
wood. Common causes of iron stain include use of ungalvanized or poorly
galvanized fasteners, cleaning with steel wool and/or wire brush, and contact of
the wood with any iron or steel. Finishing wood with a WR or WRP greatly
minimizes iron stain.
Removal of Iron Stain
Iron stain can be removed by scrubbing the stained area with a aqueous solution
of oxalic acid in water. Oxalic acid is usually sold at drugstores and hardware
stores. Dissolve 1 to 4 oz of oxalic acid in 1 qt of hot water. Scrub stained
area using a stiff-bristle brush. Thoroughly rinse with water after treatment
[Note: 1 qt =0.9 liter; 1 oz =28 g].
Caution: Oxalic acid is toxic.
Wear rubber gloves and avoid contact with skin. Work in a well- ventilated area.
Avoid splashing the solution on plants because it can damage the foliage. Wash
hands before eating and/or using tobacco products. Store in locked space
out-of-reach of children.
Raised Grain
The wetting and drying cycle of wood exposed outdoors can raise the grain of
the wood, resulting in a rough surface. On flat-grained lumber, the raised grain
may appear as thin knife- like feathers along the earlywood-latewood interface.
This degradation leads to a splintery wood surface and eventually to checking
and cracking. Checking may increase the uptake of water, thus accelerating the
degradation process.
Decay
Whereas weathering is degradation of the wood surface, decay (also called rot)
affects the full volume of wood. Decay is degradation caused by a variety of
decay fungi that are capable of breaking down the structural components of wood
for food. The fungi tunnel throughout the full volume of the wood, degrading the
polymers that form the wood cells through a complicated biochemical process.
Since these polymers give wood its strength, considerable loss of strength
occurs long before visible damage is apparent. Wood decay fungi must have
adequate moisture to grow. Although the amount necessary for growth varies,
depending on the species of fungi, in general the wood must be near fiber
saturation for fungal growth.
Mildew
Mildew is caused by a type of stain fungi, which differ from decay fungi. Mildew
is not capable of degrading the structural components of wood; therefore, it
does not cause a decrease in wood strength. Unlike decay fungi, mildew fungi do
not tunnel through the wood, but live only on the surface. Like decay fungi,
mildew fungi often flourish when excessive water is present. Moisture also
encourages the growth of lichens and other micro-organisms that discolor the
wood surface. Wood can develop mildew growth rapidly, particulariy if treated
with linseed oi1 or other natural oils, which form a food source for the fungi.
Because the conditions that favor the growth of mildew, fungi also favor the
growth of wood-degrading fungi; be suspicious if wood has mildew or other
discoloration.
Mildew can affect all species of
wood, including naturally decay-resistant species and wood treated with CCA.
Some extractives are food for mildew. Thus, species with high extractives
content are slightly more susceptible to discoloration by mildew.
Mildew fungi are objectionable
because they discolor the wood. The most common discoloration is an overall
gray. Mildew can also appear as black blotchy stains. Mildew can be removed with
a liquid household bleach1-water solution. Better yet, periodic treatment of the
wood with a WRP can prevent mildew.
Advantages of Water-Repellent Preservatives
WR/WRP Formulations
Water repellents and WRPs are relatively simple wood treatments that slow
the uptake of water and help keep wood dry. The only difference between these
preservatives is that WRPs include a fungicide or mildewcide. Otherwise, the
composition of WRs and WRPs is similar: both contain 10-20% binder such as
varnish resin or drying oil (linseed or tung oil), a solvent, and a substance
that repels water (usually a wax). The oil or varnish resin penetrates the wood
surface and cures to partially seal the wood surface. The oil or varnish also
helps to bind the fungicide/mildewcide and water repellent to the wood surface.
Solvents include organic liquids such as turpentine, naphtha, and mineral
spirits or water. The amount of water repellent varies among brands. Some WRs
and WRPs are formulated with a low concentration of water repellent so that they
can be used as a pretreatment for other finishes (about 1 % by volume). Others
are formulated with a high concentration of water repellent (about 3% by volume)
and are meant to be used as stand-alone finishes.
Preservatives
The chemical preservatives used in WRPs deserve special mention. They should
not be confused with the preservatives used for pressure treating wood, such as
OCA. The term preservative is used because the WRP chemical is a registered
pesticide. These preservatives are moderately effective fungicides/mildewcides
and give some decay resistance to wood in above-ground exposure. The chemical
treatments described here are contained in the finish and are formulated for
brush application. They are not available except as formulated in a finish.
Commonly available preservatives are described in the following list.2
3-Iodo-2-propynyl butyl carbamate (commonly called
Polyphase) is
currently used in several commercial WRP formulations and pigmented stains. It
is available in both solvent- and waterborne systems at approximately 0.5%
composition by weight.
2-(thiocyanomethylthio) benzothiazole
(TCMTB) is used alone or in
combination with methylene bis (thiocyanate) (MTC or MTB). This preservative can
also be effective as a fungicide for WRP and stain formulations. It is available
in both solvent- and waterborne systems at approximately 0.5% composition by
weight.
Zinc naphthenate is available commercially in WRP formulations and
possibly in some new stains, in both solvent- and waterborne formulations.
Approximately 2% concentration by weight of zinc metal is recommended.
Copper naphthenate is available commercially in
WRPs. Solutions and
treated wood are bright green; treated wood weathers to pale green--brown in
full sunlight. This preservative is available in solvent- and waterborne
formulations at approximately 2% concentration by weight of copper metal. It is
one of the few fungicides used to pressure-treat wood for below-ground decay
resistance; it is very effective for use on cut ends of posts before being
placed in the ground.
Copper-8-quinollnolate is available in commercial WRPs and may be
available in stains. This preservative imparts a green-brown color to the wood.
Effective concentrations range from 0.25% to 0.675%.
A mixture of bis (tributyltin) oxide and N-trlchloromethylthlo phthallmlde
(the latter also commonly called Folpet) is in a number of commercial stain
formulations at 0.5% to 1.0% composition by weight.
Pentachlorophenol (penta) was used quite extensively in WRP
formulations until about 1980. It is no longer readily available to the consumer
in either the ready-to-use (5% penta) or the concentrated (40% penta)
formulation because of its high toxicity and status as a carcinogen. The use of
pentachlorophenol is controlled and restricted to registered pesticide
applicators.
Some European commercial formulations available in the United States may
contain preservatives other than those listed here. Treatments containing
borates are also being marketed as preservatives for wood products.
The word preservative is a general term that includes many different
chemicals used on wood to make it less susceptible to attack from a variety of
organisms. These organisms include fungi of various types, including stain and
decay fungi, insects, and marine borers. Some of these chemicals are effective
against a range of organisms. Others are very specific and protect wood from
only one type of organism. No single chemical will protect wood from all
degrading organisms.
Terms often used interchangeably with preservative include pesticide,
fungicide, insecticide, and mildewcide. These chemicals are all preservatives
but they protect wood from specific organisms. Fungicides protect wood from
wood-degrading fungi. Many fungicides will also protect wood against mildew. In
turn, mildewcides can sometimes provide protection against decay fungi, but they
are most effective against mildew and similar staining fungi. The chemicals used
to formulate WRPs are generally fungicides and impart resistance to both mildew
and decay fungi.
Changes in WR and WRP Formulations
Penetrating finishes were traditionally formulated using organic solvents as
carriers for the binder and water repellent. The organic solvents facilitated
the absorption of these components, thus giving a penetrating finish. The
formulations were relatively simple solutions of binder, wax, and fungicide.
About 1980, WRP formulations started to change because of concerns about solvent
evaporation from these finishes in urban areas with smog. A number of waterborne
formulations were introduced. In addition, several manufacturers began marketing
low volatile organic compound (VOC) formulations for use in some areas. (VOC is
a general term for volatile solvents and co-solvents used in both solvent- and
waterborne finishes).
Several areas in the United States have legislation in place or pending that
will limit VOC levels in architectural finishes. Recent legislation in
California, Arizona, Texas, New York, New Jersey, and several other states has
restricted the amount of organic solvent used in finishes. This legislation has
had the greatest effect on the formulation of penetrating finishes such as WRs,
WRPs, and semitransparent stains. Under provisions of the New Clean Air Act
(1990), more stringent regulations that will affect paint formulations are
currently being developed. Formulations of finishes will continue to change to
meet these regulations. The restriction of solvents has caused manufacturers to
reformulate penetrating finishes by either removing solvents to give high-solid
finishes or by relying on waterborne systems.
Penetrating Characteristics of WRP Formulations
There is considerable variation in the penetrating characteristics of
waterborne formulations. Many waterborne formulations absorb into the wood in
the same way as solventborne formulations, but others tend to form thin surface
films. The manufacturer's product literature may indicate the absorption
characteristics of the finish.
In high-solid formulations containing large amounts of natural or synthetic
oils, the proper absorption of the finish can be hampered by the sheer volume of
oil on the surface. If the oil is a drying oil, it may dry before absorbing into
dense areas, such as wide latewood bands on flat- sawn lumber. The resulting
film will appear as shiny areas on the surface. Some WRPs are formulated with
nondrying oils that act as solvents (such as paraffin oil). These oils penetrate
the wood, but do not dry. They protect the wood from degradation and mildew
attack as do other types of WRPs. Since the oils do not dry, the wood surface
may remain oily until the finish absorbs. Absorption usually takes several days,
depending on the application rate and porosity of the wood. Since the oil does
not dry, there is the possibility of tracking it indoors, if this finish is used
on decks. These products are easy to apply to decks and have about the same
durability as other penetrating clear finishes.
Preparation of WRPs
Directions for mixing a WRP were published in a Research Note in 1978.3 This
formulation was once possible because of the availability of pentachlorophenol (penta).
This pesticide is no longer available to the consumer. In addition, the
fungicides listed in this Research Note are usually unavailable. Therefore, It
is not possible to mix your own WRP.
Resistance to Decay and Mildew
Water repellents and WRPs are
effective when used on wood exposed outdoors above-ground. In areas where decay
is a serious problem or where wood will be in contact with the ground (wood
foundations or fence posts, for example), wood will need far more protection
than that afforded by surface treatment with a WR or WRP. In such cases, wood
properly protected by treatment with a commercial preservative is recommended.
Such pressure-treated wood is normally available at a lumber yard and should
conform to recognized standards for maximum service life.
For naturally decay-resistant wood
species, WRPs provide mildew resistance for both the heartwood and sapwood; if
the lumber contains portions of sapwood, treatment with a WRP is essential to
provide above-ground decay resistance.
Treatment with CCA provides
resistance to decay only, not mildew. Treatment of the preservative-treated wood
with a WPR provides resistance to mildew. The WRPs also provide above-ground
decay resistance for either sections of the wood that did not take the
preservative treatment (heartwood) or the interior of large cross-sections
exposed by cutting or drilling.
A number of commercial wood
treaters are using a combined WR-CCA treatment for 5/4 x 6 in. radius (32 x 152
mm) edged decking. This lumber is marketed under trade names such as Ultrawood,
Wolman Extra, MELCO, and Weathershield. This dual treatment gives the wood more
resistance to weathering. Since the process is quite new, the durability of the
WR treatment is not yet well-established. Although the WR is supposed to
thoroughly saturate the wood, the wood may nevertheless require periodic
maintenance with a WRP. In addition, the commercial preservative treatments do
not contain a mildewcide, so added treatment with a WRP is necessary to prevent
mildew growth. Of the treated wood currently available, these treatments should
improve the wood characteristics and extend the product service life,
particularly with regard to weathering (i.e., surface checking, cracking,
splitting, and erosion).
Safety
Care should always be exercised in
applying a WR or WRP. The solventborne formulations are volatile, flammable
mixtures. Do not breathe their vapors or expose the solutions to flame or
sparks. It is wise to wear protective clothing on the hands and arms and to take
care that the solution is not splashed in the eyes or on the face. Be especially
careful using a WRP because these solutions contain fungicides, some of which
are toxic.
If rags, paper towels, and
uncleaned brushes andlor rollers containing finishes are not disposed of
properly, they can spontaneously ignite. Store finishes in original containers
under lock and key, out of reach of children and pets, and away from foodstuff.
Use all finishes selectively and carefully. Follow recommended practices for the
disposal of surplus preservatives and preservative containers. Immerse
finish-contaminated materials in water, then seal in plastic or an empty paint
can until they can be disposed of by proper incineration or burying.
Caution: Wood preservatives can be
injurious to people, animals, and plants. Therefore, for safe and effective
usage, it is essential to follow the directions and heed all precautions on
container labels. The application of preservatives using any spray method can be
especially hazardous and extra precautions must be taken. Avoid spraying
whenever possible.
Do not use any preservatives
indoors unless they have been specifically approved and recommended for such
use.
Application of Finish
A WR or WRP can be applied to all
exterior wood that is normally painted; it can be used as a pretreatment for
paint or as a stand-alone finish. The stand-alone finishes generally have a
higher wax content. If you use a WR or WRP as a pretreatment for paint, be sure
to read the manufacturer's recommendations. Not all WRs and WRPs can be used as
prefreatments for paints.
Water-repellent preservatives are
usually intended for exterior use because the preservatives or fungicides in
them are toxic to humans, animals, and plants. Solvents and other additives may
also be harmful. It is important to read the label on the original container
carefully to determine if the material is allowed and recommended for indoor
use. When in doubt, consult the manufacturer to determine which fungicide was
used in the WRP and whether it is appropriate for your proposed use.
Be sure to follow the
manufacture's directions for temperature limitations because finishes do not
cure properly if the temperature is too low.
Procedures for Unpainted Wood
Water repellents and WRPs can be
applied to wood by brush, roller, or spray, or the wood can be dipped into the
finish. A WRP can be used as a natural finish on many wood species to help
maintain their natural appearance. For example, finishing western redcedar with
a WRP brings out the golden-tan color of the wood. This finish is not
recommended for exterior or brushed plywood. However, It is very useful for low
grades of lumber that do not hold paint well.
Treatment with a WR or WRP will be
more durable on weathered or roughsawn surfaces because such surfaces absorb a
greater quantity of the finish than does a smooth surface. During the first few
years of exposure, the natural color of the wood can be partially restored by
scrubbing the surface with a wood brightener-water mixture, such as aqueous
solutions of household bleach or oxalic acid or a commercial wood cleaner. Scrub
the wood with a stiff bristle brush and rinse thoroughly with water. Allow the
wood to dry for several days before refinishing. However, if the wood has been
exposed for several years without a finish, it may not be possible to restore
the wood to its original color. Using a wood brightener may return some original
color and remove the mildew. The amount of color that returns depends on how
much the surface has weathered.
Regardless of whether the wood is
unfinished or finished, particular care should be taken to apply a liberal
amount of the WR or WRP to the ends of boards, at joints between boards, and to
all newly exposed wood such as drill holes. Capillary flow will cause water to
climb the back of bevel siding from the lap joints. This flow of water can be
prevented by applying WR or WRP to the lap joints. In addition, the finish
should be applied to the butt ends of horizontal siding; edges and top and
bottom ends of vertical siding; and edges and corner joints in window sash,
sills, window frames, doors, and door frames. Bottoms of doors and window sash
are often overlooked. These are areas where water can penetrate deeply and cause
extensive damage if the wood is not treated. Treatment with a WR or WRP will
eliminate many such problems. . New Wood---Applying WR or WRP solution to the
surface of unfinished wood by brushing or dipping is an effective treatrnent for
siding and exterior millwork (doors, window sash, door and window frames, sills,
moldings, and fascia), wood fencing, and lawn furniture. Millwork is often
dipped in a WRP during manufacture to improve its durability. If treated
millwork has been purchased, only freshly cut surfaces need to be brush- or
dip-treated. Dipping is more effective. Care should be taken to treat ends of
boards and joints between boards.
Reflnishing--The weathering of
wood finished with WRs and WRPs is similar to that of unfinished wood. The
surface of the wood degrades, but at a slower rate than that of unfinished wood.
Timely refinishing is essential to avoid excessive wood degradation. Smoothly
planed wood surfaces often require cleaning and retreatment after the first year
of exposure. After this maintenance, refinishing is required only when the
surface starts to show uneven discoloration or small black spots, which indicate
mildew.
Specific Applications
Decks--A WRP is an
effective finish for a fully exposed deck. Although the deck will need to be
refinished frequently, there is no need for laborious surface preparation, as is
required by film-forming finishes. Annual refinishing can be done quickly using
a brush, roller, or pad. Brush application works the finish into the wood better
than do the other methods. The finish should be applied liberally to decay-prone
areas around fasteners and end-grain.
Treated Wood--Wood that has
been pressure4reated with waterborne chemicals such as CM can easily be finished
with a WRP if the wood is clean and reasonably dry. If the wood is still
waterlogged from the preservative treatment, it should be allowed to dry for
several days once the structure is built. During summer weather conditions, this
is usually enough time for the wood to dry sufliciently to accept a WRP.
Marine Uses--Docks and
similar structures in marine environments are particularly susceptible to rapid
weathering and decay. Treatment with a WRP helps preserve wood in this
environment.
Fences--Like decks, fences
are fully exposed to the weather. Many fences are left to weather naturally. If
a finish is desirable, a penetrating finish such as a WRP or semitransparent
stain that contains a mildewcide should be used. Periodic treatment with a WRP
can slow weathering and decay, thus prolonging the life of the fence. In
addition, a WRP will preserve the natural weathered appearance of the wood.
Roofs--Although wood
shingles and shakes on standard buildings have been replaced by composition and
asphalt-based shingles to a great extent, their use is still widespread in
certain areas of the country and on expensive homes. Wood shakes and shingles
are often left to weather naturally if they are made from durable species such
as western redcedar. Depending on exposure and climate conditions, the wood
generally turns silver, dark gray, or dark brown. However, in warm, humid
climates common to the southern United States and on heavily shaded roofs,
mildew, moss, and lichens can occur. These conditions are also conducive to
decay. A WRP protects the wood while preserving the natural appearance. It is
best to dip-treat the shakes or shingles before they are installed so that the
backs and butt-ends absorb the finish. The finish may be applied by dipping the
shingles to at least two-thirds their length and then letting them stand
vertically until the finish has dried.
Pretreatments for Painted Wood
New Wood--Water repellent
and WRP formulations for use as pretreatments for paint have less wax or other
water repellents compared with those formulated for use without paint. When used
as a pretreatment before painting, a WRP can be applied in the same way as when
used as a natural finish. Freshly treated wood must be allowed to dry. If the
treatment is applied with a brush, allow 2 days of drying in warm weather before
painting. If the wood is dipped for 10 or more seconds, 1 week of drying is
necessary before painting. If enough time is not allowed for most of the solvent
to dry from the wood and for the wax to absorb, the paint applied over the
treated wood may not cure or bond properly. Open joints, such as in siding,
miliwork, and facia, should be caulked after treating with a WR or WRP but
before priming.
Refinishing--When applying
a WR or WRP to previously painted wood, loose paint must be removed; the WR or
WRP should be brushed into the joints and unpainted areas. Remove excess WRP
from the painted surfaces with a rag. Allow 3 days of drying in warm weather
before repainting.
Removal of Mold and Mildew
If mildew is present, pretreat the
wood with a commercial cleaner or a chlorine bleach-water solution. Allow the
wood to dry for 1 or 2 days before refinishing.
Removal of Mildew
Commercially available wood
cleaners work quite effectively to remove mi~dew and other stains on wood. A
mildew cleaner can also be made by dissolving 1 part liquid bleach and some
powdered detergent in 2 to 4 parts water.
Suggested formula:
1/3 cup household detergent
1 quart (5%) sodium hypochlorite (household bleach)
3 quarts warm water
[Note: 1 cup =0.2 liter; 1 quart =0.9 liter]
CAUTION: Do not use a
detergent that contains ammonia; ammonia reacts with bleach to form a poisonous
gas. Many liquid detergents may contain other additives that react with
chlorine- containing bleach.
Service Life
The service life of WRs and WRPs
is about 1 year on exposed wood surfaces. However, WRPs are extremely easy to
reapply to some structures, such as decks. Water repellents and WRPs absorb
readily into the end-grain of lumber and can stop water absorption for many
years.
The effectiveness of pretreatment
of millwork with a WR or WRP has been confirmed in outdoor exposure studies. The
differences between WRP-treated and untreated window sash and frames are
significant after exposure for 5 years. The window sash had been dipped in a WRP
for 3 mm. This treatment is similar to those used by most millwork
manufacturers. Window units treated with only a WR were in reasonable condition.
Window units treated with a WRP having extra high water repellency had the best
durability. The combined effect of a preservative and a good water repellent was
the crucial factor for long-term durability. Untreated window units decayed
severely and actually fell from the test fence after only 6 years of exposure.
